As pointed out in U.S. Pat. No. 3,806,336 issued Apr. 23, 1974, it is known that the iron/chromium alloy system has, in its composition diagram, a "limit of metastability" or "spinodal" which is thermodynamically defined as the locus of disappearance of the second derivative of the Helmholtz free energy with respect to the composition of the system. When a high-temperature composition, which is of a homogeneous single-phase structure (.alpha.-phase), of the alloy is brought within the spinodal in a lower temperature range, it is transformed into a separated two-phase structure (.alpha..sub.1 +.alpha..sub.2), the phase separation being called "spinodal decomposition".
The decomposed alloy has a periodic microstructure generally of the order of hundreds of angstroms and which consists of two composition-modulated isomorphous phases in which one phase (.alpha..sub.1) is in the form of a fine precipitate uniformly distributed in another phase (.alpha..sub.2) which forms the matrix.
It is observed that if the first phase in such a microstructure is magnetic and the second is nonmagnetic, there results a single-domain structure whereby a highly retentive magnetic body can be obtained.
U.S. Pat. No. 3,806,336 discloses that the iron/chromium alloy of spinodal decomposition type, when it contains cobalt, optionally also with one or both of molybdenum and tungsten in the proportions set forth therein, represents an improved magnetic-material system whose magnetic retentivity and magnetic energy product are comparable with or generally even higher than those of "Alnico" (iron/aluminum/nickel/cobalt) alloys which have hitherto been the mainstay of the magnetic industry. In addition to their excellent magnetic properties, the improved alloys have, because of their constituent metals, the advantages of lower material cost and better workability than the conventional alloys. It has also been taught that addition of silicon up to a certain proportion moderates heat-treatment conditions required to accomplish the spinodal decomposition of the alloys without materially decreasing the desirable magnetic properties attainable therewith. The art has also recognized that the addition of one or more of manganese, nickel, copper and aluminum in a small proportion may be advantageous.
As noted above, the desirable magnetic characteristics of the alloy are imparted when the high-temperature homogeneous single phase i.e. .alpha.-phase, decomposes into the two isomorphous phases, i.e. .alpha..sub.1 and .alpha..sub.2 phases, through the spinodal.
Accordingly, the method of preparing a magnetic body of the improved alloy system essentially comprises the procedures required to effect the spinodal decomposition of the alloy of a preselected composition.
The composition may be prepared by melting constituent metals or components together in a suitable furnace or crucible and then casting the melt to form ingots. While such ingots may, after machining to a suitable dimension, be subjected directly to the treatment procedures, it is possible to convert the alloyed ingot into a powder and then to compact and sinter the particles to a coherent body of a desired geometry.
In order to effect the spinodal decomposition, while a gradual cooling may be employed to pass the alloy from the high-temperature phase through the miscibility gap area, the following steps have been found more practical and highly suitable. The initial step comprises a solution treatment which includes heating at an elevated temperature for a substantial period of time and subsequent quenching to bring the homogenized high-temperature .alpha. phase to room temperature. The quenched body is then tempered or aged whereby the spinodal decomposition to .alpha..sub.1 and .alpha..sub.2 phases is obtained. The solution treatment may be preceded by hot or cold working. The tempering is preferably done stepwise at different temperatures. The solution-treated body is preferably subjected to an isothermal treatment in a magnetic field prior to the final tempering treatment. Magnetic properties of the body are generally improved when a cold working step is used prior to the final quenching step and subsequent to a preliminary tempering step or the magnetic treatment step.
With the prior compositions, however, to accomplish the solution treatment successfully and thus to form the homogeneous single phase .alpha. and bring the same to room temperature or aging temperature requires heating to as high as 1300.degree. C. and subsequent quenching at a cooling rate as high as 200.degree. C./sec. Heating to such a high temperature is also required when hot working of the alloy ingot is to be done preparatory to the solution treatment. While, as taught in the above-mentioned U.S. patent, the quenching rate can be lowered substantially by having the alloy contain silicon in the range as specified, the high-temperature heating requirements have imposed great difficulties on the manufacturing process and left much to be desired with respect to the economy of the produced magnets.
In order to overcome these difficulties, efforts have been made to explore a further component or components effective to extend the domain of the homogeneous .alpha. phase of the alloy system thereby enabling the alloy to be solution-treated and hot-worked at a lower, more practical temperature than the conventional composition while retaining excellent magnetic properties and an improved cold-workability. Thus, for example, in U.S. Pat. No. 3,954,519 issued May 4, 1976, there has already been disclosed an improved spinodal decomposition type alloy of the class described which by weight consists of essentially 3 to 20% cobalt, 10 to 40% chromium, 0.2 to 5% one or both of niobium and tantalum, 0 to 5% aluminum and the balance iron. As taught therein, the addition of one or both of niobium and tantalum, preferably also with 0.2 to 5% aluminum is effective to extend the domain of the .alpha. phase while reducing the .gamma. phase of the alloy system, thus making it possible to accomplish the solution treatment at a temperature as low as 900.degree. C. or even in the order of 650.degree. depending upon the relative alloy compositions.
The above proposed alloy, however, still has drawbacks arising from the fact that, for it to be effective or for better results, it commonly requires the addition of aluminum besides niobium and/or tantalum. A melt of the alloy added with aluminum give rise to handling difficulties for casting and tends to yield irregular products. Moreover, while the use of best process parameters and compositions has allowed the alloy to achieve a maximum energy product as high as 5.7.times.10.sup.6 G.Oe (with cold working) and 4.7.times.10.sup.6 G.Oe (without cold working), the magnetic performance typically attainable by procedures currently adoptable for mass production purposes is limited to 4.times.10.sup.6 G.Oe or less and cannot be said to be satisfactory.